The main function of packages is the preservation and protection of all types of products, with foods and raw materials the field of highest priority. These products require attention given the contamination generated by microorganisms (bacteria, spores, fungi, etc.) during manipulation (Tharanathan, 2003). Protection is made through packages, which are generally elaborated from synthetic polymers. Nevertheless, the indiscriminate use of synthetic packages has generated serious ecological problems, contributing to environmental contamination provoked by solid wastes of low degradability, which has driven the search for natural biopolymers. Availing of natural resources as a source of conservation and recycling becomes an excellent option and innovation in the development of new biodegradable products. Its total biodegradation into products like CO2, water, and then into compost is a great advantage against synthetic products (Bastioli, 2001).
Total replacement of synthetic plastics by biodegradable materials to elaborate packages has not been accomplished until now; nevertheless, in specific applications some synthetic polymers have been substituted by other natural materials. Said replacements have permitted the development of products with specific characteristics related to barrier, mechanical, and thermal properties in certain packages like films, protectors, foams, wrappings, plates, cups, spoons, bags, etc., (Avérous and Boquillon, 2004; Wang et al., 2003).
Natural biopolymers come from four big sources: animal origin (collagen/gelatin), marine origin (chitin/chitosan), agricultural origin (lipids and fats and hydrocolloids: proteins and polysaccharides), and microbial origin (polylactic acid (PLA) and polyhydroxyalkanoates (PHA)) (Tharanathan, 2003).
Polysaccharides are known for their complex structure and functional diversity (Stawaski and Jantas, 2003). The linear structure present in cellulose (1,4-b-D-glucan), amylose (a component of starch 1,4-a-D-glucan), and chitosan (1,4-b-D-carbohydrate polymer) provide the films hardness, flexibility, and transparency; the films are resistant to fats and oils.
Starch is an abundant raw material, specifically that coming from corn. It has thermoplastic properties when structural disruption takes place at molecular level. The presence of amylose at 70% in amylose-corn starches gives a strong structure and more flexibility to the film. The branched structure of amylopectin generally gives the film poor mechanical properties. The compounds of hydroxypropylated starches are used for the preservation of candies, raisins, nuts, and dates to avoid oxidative rancidity (Tharanathan, 2003). Synthesis of copolymerization and grafting of monomers like acrylonitrile (AN) generate a precursor of acrylic fibers used in the preparation of starch compounds plus polymer (starch-graft-PAN), which are also biodegradable (Tharanathan, 2002).
Research on biodegradable plastics based on starch began since 1970 and currently continue in several laboratories throughout the world. Technologies still being developed are related to the incorporation of the starch granule or starch in gelatinized form to the formulations of films manufactured in processes of compression, extrusion blowing, single or double-screw extrusion and injection molding (Blacido et al., 2005; Parra et al., 2004). The problem presented by the films manufactured with starch is sensitivity to humidity, which has been reduced by using in the formulations polyvinylalcohol (PVA), glycerin, sorbitol, nitrogenous bases, etc., (Shamekin et al., 2002; Smits et al., 2003; Finkenstadt and Willett, 2004; Yu, 2004; Acosta et al., 2006). Plasticization of the native starch granule or hydrolyzed starch is obtained through the structural disruption resulting from a decrease of the crystals during the extrusion process and the action of the plasticizer, with a new type of material emerging known as thermoplastic starch (TPS) (Acosta et al., 2005; Villada, 2005). Likewise, studies have been conducted on TPS made from amylose and amylopectin; in these, the barrier properties were analyzed, showing high permeability to O2 and decrease on water vapor in amylose TPS compared to those elaborated from amylopectins (Forssell et al., 2002; Dole et al., 2004; Jansson and Thuvander, 2004; Blacido et al., 2005).
TPS is a material obtained through the structural disruption (modification) occurring within the starch granule when it is processed with low water content and the action of thermal and mechanical forces in the presence of plasticizers that do not evaporate easily during processing (Bastioli, 2001). TPS presents several attributes, besides its biodegradability, it is a flexible and renewable material and it can be easily adapted to different processes of thermoplasticization by using standard equipment used in the manufacture of synthetic polymers, like injection molding, extrusion blow molding, injection molding, molding by compression, extrusion of flat film and radiation by molding (Van Soest et al., 1996b; Zhai et al., 2003).
Adding natural polymers like starch inside the polyethylene (synthetic polymer), in granular form between 6 and 30% is another approach in the manufacture of biodegradable packaging. Films from starch and low-density polyethylene (LDPE) contain up to 30% starch, shown as a partially biodegradable material. Another application of starch is the combination in gelatinized form in proportions between 30 and 70% mixed with synthetic polymers also gelatinized, like the case of polyvinylalcohol in proportions varying between 10 and 20% (Muratore et al., 2005).
Different products exist in the market made of synthetic polymers and gelatinized starch commercialized by Mater-Bi® (Hanna, 2004). However, currently both types of materials cannot be considered completely biodegradable compounds (Pedroso and Rosa, 2005). Plastic bottles of starch formed from foams through mixtures of starch with polylactic acid are used as filler material that dampens and protects against blows and vibrations during transport (Peesan et al., 2005; Xu et al., 2005).
In spite of the advantages of materials derived from starch, TPS presents little stability when humidity conditions are high (Avérous and Frigant, 2001; Avérous and Boquillon, 2004; Avérous et al., 2003). One of the problems of using TPS in bio-plastics is its fragile nature, relatively caused by its low vitreous transition temperature (Tg) and the lack of sub-Tg relaxation due to the starch's molecular chain (Kenshi et al., 1999; Shogren et al., 1993). Additionally, eventual migration of plasticizers into the environment increases the material's fragility (De Graaf et al., 2003). Fragility is a problem of structural stability that increases over time due to diminished free volume and retrogression of starch (Kuakoon et al., 2003). To increase TPS flexibility, a wide variety of plasticizers have been used like sugar, polyalcohols, amino acids, lipids, sorbates, and phosphates (De Graaf et al., 2003; Fama et al., 2005; Kuakoon et al., 2003; Nashed et al., 2003; Parra et al., 2004; Petersson and Standing, 2005). Some studies hold that nitrogenous compounds prevent retrogression in starch gels longer than other additives, increasing the stability of the gel (Ma and Yu, 2004; Shogren et al., 1993). However, most nitrogenous compounds are in solid state and melt at high temperatures yielding little flexibility (Avérous and Frigant, 2001). Another problem in the development of TPS is the presence of high contents of amylose, which diminishes flexibility compared to TPS made from high contents of amylopectin (Van Soest and Essers, 1997). Additionally, during storage the TPS made from native starches undergo structural changes, presenting greater fragility or rigidity depending on plasticizer content (Van Soest et al., 1996a).
To summarize, against ordinary plastic polymers, thermoplastic starch presents disadvantages like: its solubility in water, high hygroscopicity, rapid aging due to retrogression, and poor mechanical properties, which limit some applications like packing. These problems have been reduced by incorporating onto the thermoplastic matrix natural fillers like cellulosic fibers that serve as reinforcement material to improve the mechanical properties: effort and elongation, properties of vital importance in evaluating any synthetic or biodegradable plastic material, given that they permit characterizing the material and its application in the development of any package (Salgado et al., 2008). These compound materials are comprised of three phases: reinforcement that provides resistance and rigidity, the matrix that is the material sought to reinforce, and the interface responsible for adequate compatibility between the matrix and the reinforcement (Tserki et al, 2005), where the quality of the interface determines the final properties of the material, permitting correct fiber-matrix adhesion, ensuring transference of stress from the matrix to the fiber (Tserki et al., 2006).
In the field of patents different publications are found related to biodegradable plastic mixtures that incorporate starch in their composition, for example, U.S. Pat. No. 586,141 reveals a biodegradable plastic composition comprising a polyethylene matrix and a biodegradable aliphatic polyester, native corn starch, potato, rice, and their mixtures or modified, a starch plasticizer, a starch de-structuring agent, a coupling agent, a radical initiator, and an antioxidant agent. Likewise, the patent claims a procedure for the elaboration of the biodegradable plastic composition comprising the stages of: (i) Feeding a mixture of polyethylene and a biodegradable aliphatic polyester in proportions 1:1 to 1:30, the coupling agent and a radical initiator through the chute of a twin-screw extruder and a mixture of starch, plasticizer, de-structuring agent, and antioxidant through the lateral chute of a twin-screw extruder; (ii) Mixing the matrix and the starch mixture, and (iii) Subjecting the mixture to reactive extrusion at a temperature from 150 to 220° C. at a screw rate of 50 to 300 rpm.
U.S. Pat. No. 6,235,816 points to a method to manufacture a biodegradable thermoplastic mixture comprised of: (i) Combining pre-dried starch and a plasticizer agent to form a molten thermoplastic starch with water content below 1%; (ii) Combining at least one polymer selected from the group comprising: aromatic polyesters, polyester copolymers of aromatic and aliphatic blocks, polyester amides, polyethylene oxide polymers, polyglycols, and polyester urethanes with the molten thermoplastic starch and an aliphatic polyester (PLA, PCL, polyhydroxybutyric acid or copolymer of polyhydroxybutyric and hydroxyvaleric acid), where the thermoplastic starch comprises between 10 and 95 weight % of the starch/polymer mixture and the stage is carried out at one or more temperatures in the range from 120 to 260° C., preferably between 140 and 160° C.; (iii) Solidifying the mixture in water and permitting the mixture to reabsorb water to a content in the range of 1 to 6 weight %.